World number one Serena Williams emerged from her toughest test at this year’s US Open as she battled past Romania’ Simona Halep to book her place in the semi-finals.

Serena is aiming for her seventh US Open title and to surpass Steffi Graf’s Open era record of 22 Grand Slam titles. (Source: AP)

World number one Serena Williams emerged from her toughest test at this year’s US Open as she battled past Romania’ Simona Halep to book her place in the semi-finals of the women’s singles event here at Flushing Meadows On Thursday.

After winning the first set, the American failed to capitalize on it and went down in the second before she eventually edged past Halep to register a 6-2, 4-6, 6-3 win in the last eight contest, that lasted two hours and 14 minutes.

“I knew that I could play a lot better. I knew that I lost my rhythm and that I would have to step it up in the third. But I’m happy to be in the semifinals,” Serena was quoted as saying by US Open.org after her second-set letdown, in which she amassed 25 of her 43 unforced errors.

Serena is aiming for her seventh US Open title and to surpass Steffi Graf’s Open era record of 22 Grand Slam titles.

With the win, Serena has now set up a clash against 10th-seeded Karolina Pliskova of the Czech Republic for a place in the finals.

Meanwhile, Andy Murray made his earliest Grand Slam exit of the year after suffering a shocking defeat at the hands of Japan’s Kei Nishikori in their men’s singles encounter.

The second-seed, who took lead in two sets, suffered a 6-1, 4-6, 6-4, 1-6, 5-7 defeat against Nishikori in their quarter-final clash of three hours and 57 minutes.

Nishikori, sixth seed, will next lock horns with either Juan Martin del Potro or Stan Wawrinka.